Sudan President Omar al-Bashir, who was charged with war crimes, is under pressure to end the war in Darfur.

UNITED NATIONS - The United Nations is investigating "troubling reports" of bombings and fighting in the Darfur region of Sudan in violation of a cease-fire agreed to last week, the organization said Wednesday.

"During the past few days, the United Nations has received troubling reports of aerial bombings near Kutum in northern Darfur, as well as reports of fighting in the area of Tine, western Darfur and along the border with Chad," said a statement issued by a spokesperson for Secretary-General Ban Ki-moon.

"The secretary-general takes these reports very seriously and calls on all parties to refrain from hostilities, to respect the spirit of the cease-fire recently declared by the government of Sudan and to cooperate with UNAMID [United Nations-African Union Mission in Darfur] in investigating these reports."

Last week, Sudanese President Omar al-Bashir agreed to an immediate and unconditional cease-fire in Darfur, where government forces have waged a bloody war against militias -- a war that some international critics have characterized as genocide.

The truce was one of several recommendations to come out of a month-long meeting of government officials, Sudanese political leaders, and tribal and rebel leaders from Darfur, according to Mohamed Hussein Zaroug, a Sudanese diplomatic official in London, England.

Al-Bashir announced the cease-fire November 12 in a speech in Khartoum, Sudan. He is under pressure to end the fighting, particularly since he was charged with genocide by the International Criminal Court earlier this year for the government's campaign of violence in Darfur.

Fighting in the western part of Darfur broke out in 2003, when rebels began an uprising and the government launched a brutal counter-insurgency campaign.

The Sudanese authorities armed and cooperated with Arab militias that went from village to village in Darfur, killing, torturing and raping residents there, according to the United Nations, Western governments and human rights organizations.

The militias targeted civilian members of tribes from which the rebels drew strength.

In the past five years, an estimated 300,000 people have been killed through direct combat, disease, or malnutrition, according to the United Nations.

Another 2.7 million people have been forced to flee their homes because of fighting among rebels, government forces, and the allied Janjaweed militias.